The present invention relates to the tread of a highway tire having a radial carcass reinforcement and a crown reinforcement and, more particularly, to a tread of that type intended for travel on snow-covered, icy or wet ground.
Such a tread is generally formed of elements in relief (ribs or blocks) which are separated from each other in the circumferential direction and/or transverse direction by grooves and have numerous incisions or slits, the width of which, other than zero, is far less than the width of the circumferential and transverse grooves which delimit the elements in relief. The width of an incision is generally variable as a function of the size of tire in question, but is usually between 0.1 mm and 2 mm.
Slits or incisions normal to the surface of the tread can be used. However, the improvement in adherence on snow or ice due to these normal slits disappears during the life of the tread. In order to overcome this problem and thus impart greater effectiveness to the tire, whether upon being driven or upon being braked and whatever the wear of the tread, U.S. Pat. No. 4,298,046 provides the elements in relief in the lateral zones of the tread with incisions of substantially transverse orientation on the tread but inclined with respect to the normal to the surface of the tread by an angle of at most 45.degree. in the direction of rotation of the tire, and provides the elements in relief in the central zone with incisions inclined in the direction opposite the direction of rotation. The described solution imposes a direction of rotation on the tire, the tread of which is provided with such incisions.
In order to eliminate the imposed direction of travel and improve the aspect of the tread upon wear, U.S. Pat. No. 4,794,965 describes a tread comprising blocks having an even number of incisions, these incisions having in cross section, over their entire depth, broken-line contours referred to as "in phase opposition".
In order to impart to the tire the best effectiveness possible for longitudinal adherence on the types of ground in question under driving torque or braking torque, both of the solutions described above employ a large number of incisions and a modification, during travel, of the configuration of the edges delimiting the slices of rubber present between the incisions, the number of edges remaining constant as a function of the rate of wear.
Whatever the radial outline of the incisions, a substantial deficiency in adherence on the types of ground in question appears, beginning at a given rate of wear of the tread of between 30 and 55%. This phenomenon is particularly evident on radial tires, the tread of which is stabilized by a poorly deformable reinforcement.
This deficiency is perceived by the user all the more rapidly the greater the number of incisions on the tread, this number of incisions being then the ground of a high rapidity of wear of the tread, accentuated furthermore by the deliberate selection of radial incision contours causing irregular and self-maintained wear of the rubber slices delimited by the incisions. This rapidity of wear has the result that the rate of wear cited above is reached at the end of a number of miles which is undoubtedly small as compared with the mileage achieved by a normal tire, that is to say, one which is not entirely cut, for the same rate of wear.
The problem forming the basis of the invention is first of all to remedy the lack of adherence on the types of ground in question during the life of the tire, and secondly, to decrease the rapidity of wear of the tread while retaining good adherence of the tire on the types of ground in question.